1. Field of the Invention
This invention relates to a system for controlling the locomotion of a legged walking robot and, more particularly, to a system for controlling the locomotion of a biped walking robot or the like which compliance-controls the leg links at the time of footfall to so as to achieve good terrain adaptability.
2. Description of the Prior Art
The considerable impact a walking robot receives from the ground when its free foot touches down is large enough to destabilize the robot's locomotion. For cushioning footfall impact (footfall being the event of placing or landing of a foot) the applicant previously proposed in Japanese Patent Application No. 1-297199 a system in which means for detecting the ankle moment caused by the impact with the ground is installed in the linkage of each leg and a speed command proportional to the magnitude of the detected moment is applied to an ankle drive motor for achieving a ground compliance operation. This system is based on the concept of virtual compliance control involving impedance control implemented using speed analysis control. [See "Virtual compliance control of multi-degree-of-freedom robots" (Collected papers of the Measurement and Automatic Control Society, Vol. 22, No. 3, March 1986.)] Although the proposed system makes a major contribution to stabilization of robot locomotion by enabling the impact at footfall to be absorbed to a satisfactory degree, it nevertheless still has the following drawback.
Even when the shock received upon landing of the feet becomes large, as during fast walking or when the robot is carrying a heavy object, the impact at the instant of footfall can still be adequately absorbed insofar as the compliance operation is quick, but since the speed with which the bottom of the foot strikes the ground under such circumstances is high, the foot tends to act as if it were trying to kick the ground so that, from an overall viewpoint, the impact absorption effect becomes worse rather than better. Slowing the compliance operation to avoid this problem does not help because this makes it impossible for the impact absorption to be carried out smoothly at the moment of footfall and thus causes the absorption of the impact to become insufficient. Therefore, insofar as the compliance speed is maintained constant throughout the footfall event, the impact absorption will be insufficient during fast walking, walking while carrying a heavy payload and the like, no matter what compliance speed is selected.
The first object of this invention is therefore to provide a system for controlling the locomotion of a legged walking robot whose system for controlling the compliance operation is able to provide control covering the full walking speed range from slow to fast and can achieve improved impact absorption irrespective of whether or not the robot is carrying a heavy payload.
In the control system according to the applicant's earlier Japanese patent application mentioned above, the switchover between the control of the compliance operation for absorbing and cushioning footfall impact and the ensuing position control in which no compliance operation is conducted is discontinuous and this discontinuity impairs smooth walking. For example, switchover occurring when a toe or heel is just above ground causes the toe or heel to kick against the ground, whereby the system generates an impact that would not otherwise occur.
The second object of the invention is therefore to provide a system for controlling the locomotion of a legged walking robot in which the transition from impact absorption control to position control is carried out smoothly.
Moreover, in the control system according to the earlier patent application the impact absorption control is initiated only after a footfall signal has been received. Thus in cases such as where the leg linkage of the free leg comes in contact with something projecting above the ground, it is not possible to realize the desired flexible "willow-in-the-wind" control of the ankle because the system is conducting position control at this time. If the ankle should be able to yieldingly adapt to external force at such times, the impact received by the robot would be considerably moderated and the danger of toppling would be greatly diminished.
The third object of the invention is therefore to provide a system for controlling the locomotion of a legged walking robot which is capable of also controlling the ankle of the free leg for coping with situations such as that just described.